1. Field of the Invention
This invention relates to a semiconductor optical modulator, more specifically to an ultra-high speed semiconductor optical modulator with a traveling-wave electrode which operates at a low driving voltage, with reduced loss and in ultra-wide bandwidth.
The present invention can be applied to a future ultra-high capacity optical transmission system to be used in the U.S. and Europe.
2. Description of the Prior Art
High-speed semiconductor optical modulators studied for future high-density transmission systems can be categorized into two types. One is an electro-absorption (E-A) optical modulator with a lumped-element (L-E) electrode. The other is an electro-optic (E-O) optical modulator with a traveling-wave (T-W) electrode. The following will discuss a semiconductor electro-absorption optical modulator with a lumped-element electrode and a semiconductor electro-optic optical modulator with a traveling-wave electrode.
FIG. 1 shows a bird-view of a conventional semiconductor electro-absorption optical modulator with a lumped-element electrode. In this FIG. 1 is the p-electrode, 2 is the p.sup.+ -InGaAs cap layer, 3 is the p-InP cladding layer, and 5 is the core. Here, an i-InGaAlAs(13 nm)/InAlAs(5 nm) multi-quantum well (MQW) core is assumed to be used for a core. Six is a n-InP cladding, 7 is a n-InP substrate, 8 is a n-electrode, and 9 is a polyimide. Here, the electrode on the polyimide 9 is called a bonding pad and 10 is a bonding wire. The electric signal supplied by a signal generator is applied to the bonding wire 10.
In order to explain the operation principle of a semiconductor electro-absorption optical modulator, an optical absorption spectrum of the MQW core 5 is shown in FIG. 2. The optical absorption peak is set at around 1.49 .mu.m for a signal light of 1.55 .mu.m. As shown in this figure, since the difference between the wavelength of the operating signal light and the wavelength of the absorption edge is large for the zero-biased condition (solid line A), the incident wave can be emitted without suffering significant absorption. Thus, the ON-state can be achieved. On the other hand, since the absorption spectrum is shifted toward the long wavelength region under the reverse-biased condition (broken line B), the signal light is absorbed in the i-MQW core, 5. This results in the OFF-state.
The p-electrode, 1, of this conventional semiconductor optical modulator is used for lumped-element operation. In order to explain the operation, FIG. 3 shows an equivalent circuit, which includes a driving signal generator. Here, S.sub.G is a driving signal generator, R.sub.G is its characteristic impedance, R.sub.L a termination resistor, C.sub.MQW is the capacitance of the i-MQW core, 5, and C.sub.P is the capacitance of the above-mentioned bonding pad. The electrical 3-dB bandwidth, .DELTA.f.sub.el, for this structure can be approximately expressed as, EQU .DELTA.f.sub.el =1/(.pi..multidot.R.sub.L .multidot.C.sub.MQW).(1)
Generally, the termination resistor R.sub.L has the same characteristic impedance of 50.OMEGA. as that (R.sub.G) of the driving signal generator S.sub.G. Here, we assumed that since the capacitance of the pad is sufficiently small, the total capacitance can be determined by the capacitance of the i-MQW core, 5. When we assume that the thickness (d), width (W) and length (L) of the i-MQW core, 5, are respectively 0.2 .mu.m, 2 .mu.m and 300 .mu.m, the capacitance of the i-MQW core, 5, can be obtained from the following equation, EQU C.sub.MQW =.epsilon..sub.0.multidot..epsilon..sub.r .multidot.W.multidot.L/d. (2)
Here, .epsilon..sub.0 and .epsilon..sub.r are respectively the dielectric constant of the vacuum and the relative dielectric constant of the i-MQW core, 5.
From the Eqs. (1) and (2), the electrical 3-dB bandwidth, .DELTA.f.sub.el, for the above-mentioned semiconductor lumped-element optical MQW modulator is around 20 GHz or less. Although the electrical 3-dB bandwidth, .DELTA.f.sub.el, can be improved by using smaller value of C.sub.MQW, the extinction ratio of the signal light is degraded. When we assume that .DELTA. is the increase of the absorption coefficient and .GAMMA. is the confinement factor of the propagating field into the i-MQW core, 5, the extinction ratio, R, of the signal light can be expressed as, EQU R=exp(-.DELTA..alpha..multidot..GAMMA..multidot.L). (3)
As shown in this equation, a too short i-MQW core, 5, degrades the extinction ratio. Thus, a too short i-MQW core, 5, cannot be used from the view-point of the extinction ratio.
As explained above, there is a severe trade-off between the electrical 3-dB bandwidth .DELTA.f.sub.ef, which is limited by the CR-constant, and the extinction ratio. Thus, there is a great difficulty for realizing an ultra high-speed semiconductor optical modulator with a 50 GHz modulation bandwidth and a high extinction ratio.
The conventional semiconductor optical modulator with a traveling-wave electrode makes use of the electro-optic effect which means that the refractive index is changed by applying the biased voltage (R. SPICKERMANN et al., IEE Electronics Letters, vol. 32, pp. 1095-1096, 1996). The semiconductor optical modulator's equivalent circuit is shown in FIG. 4. As is well known, the modulation index, m(f), can be expressed as (S. H. Lin et al., Applied optics, vol. 26, pp. 1696-1700, 1987), ##EQU1## where L is an interaction length between the electric signal and light, i. e. the length of the traveling-wave electrode. And, n.sub.o and n.sub.m are respectively the effective indexes of the optical wave and electric signal. Here, .omega.m is the angular frequency of the electric signal, C.sub.0 is light velocity in the vacuum, .alpha.m is the microwave attenuation factor, and Z is the characteristic impedance of the semiconductor optical modulator using the traveling-wave electrode.
Since the characteristic impedance of the conventional optical modulator is 50.OMEGA., the non-doped layer, which does not have intentional doping, is thick. Furthermore, the electro-optic effect, i.e. index change effect, is small even for the case of a MQW core structure. Thus, the traveling-wave electrodes have long interaction lengths of the order of millimeters.
Next, the influence of the electrode length on the modulation bandwidth will be qualitatively discussed. For simplicity, by assuming the velocity matching between electric signal and light (n.sub.m =n.sub.o) and impedance matching between a semiconductor optical modulator and outer circuits (Z=R.sub.G =R.sub.L), the following simple relation can be obtained from Eq. (4) for the modulation bandwidth, .DELTA.f, EQU .DELTA.f.varies.1/(.alpha..sub.m L).sup.2. (6)
Therefore, long traveling-wave electrodes significantly degrade the modulation bandwidth due to the electrode conductor loss.
Thus, it was almost impossible to realize a high-speed semiconductor electro-optic optical modulator with a low-driving voltage by making use of a traveling-wave electrode.
Recently, one traveling-wave semiconductor electro-absorption modulator has been reported (N. Agrawal, et al., European conf. Integ. Opt. (ECIO), 1997, Paper EFB3-1). It has a relatively short interaction length (500 .mu.m), but its modulation bandwidth was limited to 18 GHz (3-dB electrical) and no data has been reported on the driving voltage, intrinsic layer thickness, and characteristic impedance.